| | Jefferson County | IL | 17081 | | Mount Vernon, IL 34500 | | Central | nonmetro county (rural) | nonspecialized | https://maps.google.com/?q=JeffersonCounty+IL |
| | Jersey County | IL | 17083 | | St. Louis, MO-IL 41180 | St. Louis-St.
Charles-Farmington, MO-IL 476 | Outlying | metro county (urban) | nonspecialized | https://maps.google.com/?q=JerseyCounty+IL |
| | Jo Daviess County | IL | 17085 | | | | | nonmetro county (rural) | recreation | https://maps.google.com/?q=JoDaviessCounty+IL |
| | Johnson County | IL | 17087 | | | | | nonmetro county (rural) | federal/state
government-dependent | https://maps.google.com/?q=JohnsonCounty+IL |
| | Kane County | IL | 17089 | Elgin, IL 20994 | Chicago-Naperville-Elgin,
IL-IN 16980 | Chicago-Naperville,
IL-IN-WI 176 | Central | metro county (urban) | nonspecialized | https://maps.google.com/?q=KaneCounty+IL |
| | Kankakee County | IL | 17091 | | Kankakee, IL 28100 | Chicago-Naperville,
IL-IN-WI 176 | Central | metro county (urban) | nonspecialized | https://maps.google.com/?q=KankakeeCounty+IL |
| | Kendall County | IL | 17093 | Elgin, IL 20994 | Chicago-Naperville-Elgin,
IL-IN 16980 | Chicago-Naperville,
IL-IN-WI 176 | Central | metro county (urban) | nonspecialized | https://maps.google.com/?q=KendallCounty+IL |
| | Knox County | IL | 17095 | | Galesburg, IL 23660 | | Central | nonmetro county (rural) | nonspecialized | https://maps.google.com/?q=KnoxCounty+IL |
| | Lake County | IL | 17097 | Lake County, IL 29404 | Chicago-Naperville-Elgin,
IL-IN 16980 | Chicago-Naperville,
IL-IN-WI 176 | Central | metro county (urban) | nonspecialized | https://maps.google.com/?q=LakeCounty+IL |
| | LaSalle County | IL | 17099 | | Ottawa, IL 36837 | Chicago-Naperville,
IL-IN-WI 176 | Central | nonmetro county (rural) | nonspecialized | https://maps.google.com/?q=LaSalleCounty+Illinois |